15-4 What is 'Action Potential' or 'Nerve/Electrical Impulse?' (Cambridge AS A Level Biology, 9700)

Action potentials, nerve impulses, and electrical impulses are terms used interchangeably to describe the signal traveling along a neuron. This signal moves from sensory to relay to motor neurons, specifically along the axon. Unlike common misconceptions, these impulses are not electrons moving, but rather a more complex physiological event.

The textbook definition of an action potential is a 'rapid change in the electrical charge distribution across a cell surface membrane.' This definition can be confusing at first, so the video aims to break down each part to provide a clearer understanding.

Neurons, being cells, possess a cell surface membrane, specifically along the axon. This membrane, depicted as a phospholipid bilayer, is crucial to the action potential. For simplicity, it's represented as a simple line, with signals moving from point A to point B along this membrane.

The electrical charge distribution refers to the separation of charges across the cell surface membrane, with the outside typically more positive and the inside more negative. This distribution is maintained by ions like sodium and potassium. Prior to sending a signal, the axon establishes this resting membrane potential, which is essential for action potentials to occur.

When an action potential (signal) is sent, a rapid change occurs. At the point of stimulation (e.g., point A), the electrical charge 'flips': the inside becomes positive, and the outside becomes negative. This change is transient, and as the impulse moves to the next section of the axon, the previous section returns to its resting potential. This sequential flipping of charge along the axon constitutes the action potential.

Before sending a signal, the axon must establish a membrane potential where the outside is positive and the inside is negative. Upon stimulation, the charge distribution rapidly changes at that point (inside positive, outside negative). This rapid change then propagates along the axon, with each preceding area returning to its resting state. The underlying causes of these charge changes and their movement will be discussed in the next video.